This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Condenser. This is a very large lens and is used in place of the ground-glass. It consists of two plano-convex lenses mounted with the convex faces inwards. See Illustration No. 14. This lens collects all the rays of light together, passes them through the negative and brings them to a focus in the lens which projects the image. Large condensers if made of fine glass are quite expensive, but a cheaper grade can be procured, producing equally as fine results if a sheet of very fine ground-glass be mounted between the convex surfaces of the two lenses. This, however, is intended of course, only for large condensers and where electric light is used. Electric light gives a stronger light than is actually required; the diffusion caused by the ground-glass therefore is no detriment.
613. As all the light must come through the condensing lens and the negative from which the enlargement is to be made, the diagonal of the negative must not be any-larger than the diameter of the condenser; otherwise the corners of the negative would be cut off. For all negatives Up to 5 x 7, a 9-inch condenser will answer; a 10-inch condenser would be better. The latter size is more generally used than any other. The size of the condenser has nothing to do with the size of the enlargement. It has only to deal with the size of the negative you wish to enlarge from, as it collects and concentrates the light upon the negative and distributes it evenly. Any size enlargement can be made with any size condenser so long as the condenser is large enough to cover the negative from which you are enlarging. Where a condenser is used in place of the ground-glass the light, being concentrated, is so much stronger that the exposure necessary for the enlargement is reduced considerably. Any kind of light can be used with the condenser exactly the same as if the ground-glass were used. Electric light, however, is preferable to lamp light and if incandescent bulbs are used the box, as shown in the cut. can be arranged to hold the condenser in place of the ground-glass.
Enlarging Easel. On a small table or stand place a box a little larger than the size of enlargement you are going to make. Fasten this box down by some weight (place some heavy material inside) so that it will be perfectly rigid. This box will serve as an easel on which you can fasten the Bromide paper. The side of the box facing the camera should be covered with a piece of white cardboard, or cotton cloth which should be soft and perfectly smooth. A good plan, where cotton cloth is used, is to dampen it before you tack it to the box and then use plenty of tacks. When it becomes dry it will be as tight as a drum-head.
615. A more elaborate apparatus and a style usually used in the professional studios can be constructed, as shown in Illustration No. 15. The easel-board can be made to slide up and down in the frame and is held in position at any desired point by means of a flat spring attached to each side of the board and working in the groove between the board and the upright standard. The base of the easel can also be mounted on a track if desired for sliding backward and forward, nearer or further away from the camera, but this is not necessary and is not very generally used except in specially prepared enlarging rooms that are used for no other purpose. Easels may also be purchased from any photographic supply house. The Ingento Enlarging and Copying Easel is shown in Illustration No. 16, page 231.